(i) Technical Field
A certain aspect of the embodiments discussed herein is related to an optical modulator
(ii) Related Art
In an optical communication system that superimposes a signal on a light and transmits the light with an optical fiber, an optical modulator that modulates the intensity of an laser light emitted from a light source, and generates an optical signal is used. For example, a Mach-Zehnder optical modulator is widely known as the optical modulator.
The Mach-Zehnder optical modulator has a branch potion that branches an input light, two light waveguides that propagate the branched lights, a multiplex portion that multiplexes the lights propagated through the light waveguides again, and an output light waveguide that leads the output light from the multiplexer portion to the outside of the modulator. There are provided a modulation electrode and a phase adjustment electrode on the two light waveguides that propagate the branched lights. The interference condition in multiplexing is changed by a voltage applied to the modulation electrode, whereby the light output is turned on and off. The phase adjustment electrode is used to set the phase difference between the lights on the two waveguides to a predetermined value.
There is a case where a light intensity detection electrode for detecting the light intensity is provided on the output light waveguide, which follows the multiplex portion. A voltage applied to the phase adjustment electrode is feedback controlled on the basis of the detection result of the light intensity detection electrode, so that the phase adjustment can be performed accurately (see Japanese Patent Application Publication No. 7-49473).
The conventional optical modulator has the following problem to be solved. The voltage applied to the phase adjustment electrode or the modulation electrode results in a leakage current that flows through a cladding layer between the phase adjustment or modulation electrode and the light intensity detection electrode. The leakage current affects the feedback control of the phase adjustment.